The chief difficulty for documenting this in the fossil record is
finding a situation in which both lineages and the hybrids are
preserved and adequately recognizable as such.  As Harry pointed out,
precise dating is critical, but it's also necessary for whatever place
has the separate forms and then the hybrid to all get preserved and
then studied.

There are some points where variation seems to go up and then back
down in the fossil record of a species, but pinning down a cause is
tricky.  E.g., the thick, wrinkled "tridacnoides" form of Mercenaria
appearing for a while in the Pliocene of eastern North America seems
to grade into more normal ones.

--
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"

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